https://devblogs.microsoft.com/oldnewthing/20221115-00/?p=107399 Skip to main content [RE1Mu3b] Microsoft The Old New Thing The Old New Thing The Old New Thing * Home * DevBlogs * Developer + Visual Studio + Visual Studio Code + Visual Studio for Mac + DevOps + Developer support + CSE Developer + Engineering@Microsoft + Azure SDK + IoT + Command Line + Perf and Diagnostics + Dr. International + Notification Hubs + Math in Office + React Native * Technology + DirectX + PIX + SurfaceDuo + Startups + Sustainable Engineering + Windows AI Platform * Languages + C++ + C# + F# + Visual Basic + TypeScript + PowerShell Community + PowerShell Team + Python + Q# + JavaScript + Java + Java Blog in Chinese * .NET + .NET + .NET MAUI + Blazor + ASP.NET + NuGet + Xamarin + .NET Blog in Chinese * Platform Development + #ifdef Windows + Apps for Windows + Azure Depth Platform + Azure Government + Bing Dev Center + Microsoft Edge Dev + Microsoft Azure + Microsoft 365 Developer + Old New Thing + Windows MIDI and Music dev + Windows Search Platform * Data Development + Azure Cosmos DB + Azure Data Studio + Azure SQL Database + OData + Revolutions R + SQL Server Data Tools * More [ ] Search Search * No results Cancel How does Windows decide whether your computer has limited or full Internet access? [png] Raymond Chen November 15th, 20227 1 Windows lets you know when your computer's Internet connection is limited or absent entirely. What is this sorcery? Windows attempts to download a file from a dedicated Web server. Depending on which version of Windows, it's http://www.msftncsi.com/ ncsi.txt or http://www.msftconnecttest.com/connecttest.txt. If the download is successful and contains the correct contents, then Windows concludes that you have full Internet access. If something goes wrong, Windows will report either limited or no Internet access, depending on what exactly went wrong. You can read more details on docs.microsoft.com. [png] Raymond Chen Follow Tagged Tips/Support Read next What is a "Select Administrator"? Is that some special elite kind of administrator? It's two different messages that combine in an interesting way. [png]Raymond Chen November 1, 2022 10 comments Why is there a passwords.txt file on my system that's filled with somebody else's passwords? It's part of an open-source password strength package. [png]Raymond Chen October 18, 2022 10 comments 7 comments Leave a commentCancel reply Log in to join the discussion. * [png] Yuri Khan November 15, 2022 10:19 pm 0 collapse this comment Isn't this heuristic really easy to spoof? Log in to Vote or Reply + [png] Raymond ChenMicrosoft employee November 16, 2022 6:36 am 0 collapse this comment So what if somebody spoofs it? Congratulations, you tricked Windows into showing a "full internet access" icon, and then when the user tries to go to a web site, they get an error. All you're doing is adding more support costs to deal with customers complaining "Your internet is broken." Log in to Vote or Reply * [png] Simon Geard November 15, 2022 10:56 pm 0 collapse this comment Linux distros do something similar... e.g. on a current Fedora version, it's http://fedoraproject.org/static/hotspot.txt. Note that as with the Windows version, the protocol is HTTP, not HTTPS - because captive portals completely break TLS, but plaintext HTTP will result in a clean redirect to the portal, allowing the network service to detect the presence of the portal and to bring up a browser window to let the user authenticate. Log in to Vote or Reply + [png] Simon Farnsworth November 18, 2022 9:02 am 0 collapse this comment Similarly, Android and ChromeOS devices expect a HTTP 204 response from http://connectivitycheck.gstatic.com/ generate_204 (also HTTP, for the same reason). It's possibly the only time I've seen a HTTP 204 in the real world Log in to Vote or Reply o [png] Chris Warrick November 19, 2022 3:40 am 0 collapse this comment I've seen HTTP 204 in the real world a few times, REST APIs sometimes return it in response to a DELETE. Log in to Vote or Reply * [png] Douglas Dlutz November 19, 2022 12:51 am 0 collapse this comment Ok, now I'm more curious. Why two URLs split on Windows Version, why does not just using a single URL work for all versions? Also, these files must get a lot of traffic. I believe the connecttest.txt url is using Azure Front Door caching given that it has the x-msedge-ref headers (a callback to when we were called the Edge team for serving Bing traffic before we eventually grew to serve more of Microsoft and eventually be Azure product). But I don't see these headers on ncsi.txt, does that file use a different CDN? Log in to Vote or Reply + [png] Chris Warrick November 19, 2022 3:48 am 0 collapse this comment They changed the URL from "msftncsi" to "msftconnecttest". One of them makes its purpose obvious, the other is an obscure acronym. They probably did this to make it easier for people looking at their traffic to understand what this thing is. Also, do you really need a CDN to serve a 14-byte text file, even to millions of people? 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